Desired grain flow

that depends on the alloy, for some both of the anneal temps are the same.

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P.E. Metallurgy, Plymouth Tube

Annealing and stress relief are not the same. Annealing could result in grain growth wheras, stress relief is performed at a subcritical temperature and will not effect grain size.

And in some materials you can anneal, and not get grain growth.
In many cases you are also concerned about formation of secondary phases.

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P.E. Metallurgy, Plymouth Tube

thanks to everyone.
let's pick a particular material. 6061 aluminum.
let's say, the T6 condition 6061 can be thread rolled. the benefit of the rolling increases the fatigue strength of this bolt. if this bolt is subsequently subjected to high temperature, its strength will be reduced. at what temperature, the benefit of the thread rolling will be gone?

Thread rolling imposes residual stresses (strain hardening). You are asking at what temperature these stresses will start to relax. Depends on the time and temperature, generally going linear with the former and exponential with the latter. Check heat treatment handbooks. You want the ‘Stress Relief’ schedule. As to the questions in your first post, no, no, and no (but not always).

Also MMPDS-10 (formerly MIL-HDBK-5) has many graphical illustrations addressing mechanical properties vs temperature.

thanks.

Thread rolling helps in a number of ways, the strain hardening (as Jug mentioned), it also forms a smooth continuous surface with no cutting or tearing which also improves fatigue.
Many age hardenable alloys are thread rolled, right before the final aging.

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P.E. Metallurgy, Plymouth Tube

dho,

As part of the EUCAR Mg-Engine project involving Opel, Renault, Volvo, BMW and various suppliers and universities, aluminium bolts were evaluated for fatigue strength vs. temperature. Two conditions were evaluated: threads rolled after T6 heat treatment (FTR) and threads rolled before T6 heat treatment (FHT). The results were plotted as fatigue strength vs. test temperature. Fatigue strength continuously decreased from room temperature up to the maximum that was evaluated (150 °C). A 2nd order polynomial equation was fit to the data:

> Bolts M10 FHT: y = -0,0005x² - 0,0299x + 29,309
> Bolts M10 FTR: y = 0,0004x² - 0,1635x + 41,171

y axis = stress amplitude (σa50% in MPa), x-axis temperature in C

For FTR condition, stress amplitude at room temp was 38 MPa, decreased to 31 MPa at 80 °C, 27 MPa at 120 °C, and 26 MPa at 150 °C. The following is an excerpt regarding long term aging:

Ageing (2,200h/+150°C) of the bolts M10 with final thread rolling production sequence (FTR) also reduces the fatigue strength significantly from σa50% = 38 N/mm2 to σa50% = 30 N/mm2. This equals to a drop of -21%.
This result can be explained by residual stresses induced in the thread root by the workhardening effect of the final thread rolling process. The induced residual stresses increase the fatigue strength of the aluminium bolts. This effect is also known with steel bolts. The ageing at +150°C causes a reduction of the residual stresses which in consequence leads to a drop of the fatigue strength.

Source: Mg-Engine-TR-D9b-2006
Author: Th. Marx, Adam Opel GmbH, Rüsselsheim/Germany